As a result of continuous technological advances that have brought about remarkable performance improvements, light-emitting diodes (LEDs) are increasingly finding applications in traffic lights, automobiles, general-purpose lighting, and liquid-crystal-display (LCD) backlighting. LED lighting is poised to replace existing lighting sources such as incandescent and fluorescent lamps since LEDs do not contain mercury, exhibit fast turn-on and dimmability, long life-time, and require low maintenance. Compared to fluorescent lamps, LEDs can be more easily dimmed either by linear dimming or PWM (pulse-width modulated) dimming.
A light-emitting diode (LED) is a semiconductor device that emits light when its p-n junction is forward biased. While the color of the emitted light depends primarily on the composition of the material used, its brightness is directly related to the current flowing through the junction. As a result, an effective way to ensure that LEDs produce similar light output is to connect them in series so that all LEDs in a string have the same current. Unfortunately, a major drawback of the series connection of LEDs is that the cumulative voltage drop of each LED limits the number of LEDs in a string. LEDs may be placed in parallel to reduce the total voltage drop. However, often circuits placed in parallel have different currents. Thus, LEDs in parallel may output light at differing brightness. Accordingly, there is a need for a circuit to manage current flow to maintain a level of brightness between two or more LED strings placed in parallel.